This invention relates to an electrical anticorrosion device for a marine propulsion arrangement. More particularly, the invention relates to a cathodic protection arrangement which is suitable for use with such a propulsion unit.
An important group of oxidation-reduction processes are those involved in corrosion. The annual combined cost of corrosion protection and the losses due to corrosion are enormous. This fact lends a particular practical importance to this general subject matter area.
The problem of corrosion is particularly acute when related to the operation of a watercraft in an aqueous environment. Such is especially the case in a marine environment since the salt water greatly accelerates the effects of galvanic corrosion. Galvanic corrosion refers to the accelerated electrochemical corrosion produced when one metal is in electrical contact with another more noble metal, both being in the same corroding medium or electrolyte (e.g., salt water) with a current between them. Corrosion of this type usually results in a higher rate of solution of the less noble metal and protection of the more noble metal.
A number methods (often referred to as "cathodic protection" systems) have been devised over the years in an attempt to prevent galvanic corrosion of the components of various devices and arrangements. One popular method utilized in connection with various watercraft for providing cathodic protection employs an anode and a separate reference electrode wherein current is supplied to the anode to polarize a submersible metal unit, such as a marine drive unit. The potential at the material to be protected, such as the drive unit, is determined with respect to the reference electrode so that a quantity of electrical current can be supplied by an appropriate source of electrical power to the anode to establish and maintain the potential at the protected unit at desired levels which will provide optimal cathodic protection.
U.S. Pat. Nos. 4,492,877 and 4,528,460, both to Staerzl, teach such use of an anode, a reference electrode, and a current control system in connection with watercraft propulsion systems. The '877 patent discloses an electrode apparatus for cathodic protection and the '460 patent discloses a cathodic protection controller. Generally, Staerzl ('877) discloses an electrode apparatus for a cathodic protection system, for mounting on an outboard drive unit, which uses a grounded shield mounted between an anode and a reference electrode to allow the anode and reference electrode to be mounted in close proximity to each other. Generally, the cathodic protection system provided protects the lower unit of a marine stern drive from corrosion by maintaining the lower unit at a selected electrical potential (e.g., 0.94 volts). Staerzl ('460) also teaches a control system for cathodically protecting an outboard drive unit having an anode and a reference electrode mounted thereon. Current supplied to the anode is controlled by a transistor which, in turn, is controlled by an amplifier. The amplifier is biased to maintain a relatively constant potential on the drive unit when operated in either fresh or salt water.
For the sake of convenience and ease of installation of the cathodic protection system, Staerzl has recognized that it is desirable to mount the anode and reference electrode relatively close together within a single housing unit which is readily attachable to a submersible propulsion unit. In the arrangements disclosed in the Staerzl patents, the anode and reference electrode are disposed with one of these electrodes in front of the other electrode, on a line running in a direction longitudinally with respect to an associated watercraft, in an insulating housing unit securable to a bracket unit proximate to which the propulsion unit is attached. Accordingly, the distance from the anode to certain regions of the propulsion unit to be protected is different from the distance between the reference electrode and those same regions.
Although providing some measure of convenience, such an arrangement nevertheless creates certain problems. Due to the aforementioned disparity in distances, the measured potential at the point of reference may differ somewhat from the actual potential whereat anticorrosion protection is required, and which protection is meant to be secured by supplying current to the anode. These prior arrangements, thus, render it difficult to ascertain an accurate measurement of the potential at the protected regions of the propulsion unit, and therefore to supply the correct current to the anode in order to adequately protect those regions of the propulsion unit from corrosive damage. Also, it is difficult to protect against possible damage to the anode arrangement, because excessive anode current can cause damage to portions of the anode arrangement.
It is therefore a principle object of the present invention to provide an improved electrical anti-corrosion device suitable for use with a marine propulsion arrangement.
It is a further object of the invention to provide a cathodic protection arrangement which permits for the effective prevention of cathodic corrosion by insuring that a proper current is supplied to an anode.
In addition to the problem of galvanic corrosion, there is also the problem of the treatment of the exhaust gases generated during operation of a watercraft propulsion arrangement. The treatment of exhaust gases in marine propulsion units and particularly outboard drives is a troublesome one. It is well known to discharge the exhaust gases from the powering engine through an underwater exhaust gas discharge so as to utilize the body of water in which the watercraft is operating as a silencing medium. Although this is a very acceptable and effective way for silencing exhaust gases under high speed running conditions, it does present certain problems in connection with low speed exhaust gas discharge. With an outboard motor, it is the common practice to provide a separate, above the water, exhaust gas discharge which has its own silencing system for treating the idling exhaust gases. With inboard/outboard drives, on the other hand, the powering engine usually has a larger displacement and the treatment of the exhaust gases during idling presents different problems. It has been proposed with such arrangements to employ a further auxiliary exhaust gas discharge which is also underwater when the boat is traveling at low speeds but is less deeply submerged than the high speed exhaust gas discharge. Although this does provide good silencing, the exhaust gases tend to emanate in large bubbles which can cause objectionable noise.
It is therefore yet a further object of the present invention to provide an improved exhaust gas discharge for a marine propulsion unit.
It is another object of this invention to provide an improved underwater exhaust gas discharge for a marine inboard/outboard drive unit.
It is still a further object of this invention to provide an improved silencing arrangement for the idling exhaust gases of an inboard/outboard drive unit.